The present invention relates to cellular radio communications in general, and more specifically, to a method of, and apparatus for, reducing the spacing between receive antennas in a signal combining base station and using adaptive beamforming to improve the downlink performance.
In a digital cellular radio communication system, radio signals which are digitally modulated are used to convey information between radio base stations and mobile stations. The radio base stations transmit downlink signals to the mobile stations and receive uplink signals transmitted by the mobile stations. A common problem that occurs in digital cellular radio communication systems is the loss of information in the uplink and downlink signals as a result of multipath fading and interference which may exist in the radio transmission channel.
With regard to the former, multipath fading, there are basically two multipath effects: fading and time dispersion. When the path length between a mobile station and a base station is relatively short, fading arises from the interaction of the transmitted signal, or main ray, and reflections thereof, or echoes, which arrive at the receiver at approximately the same time. When this occurs, the main ray and echoes add either destructively or constructively. If there are a large number of echoes, the pattern of destructive and constructive addition takes on a Rayleigh distribution, which is why this effect is sometimes called "Rayleigh fading". Certain points in the fading pattern, where destructive addition results in fading "dips", result in a relatively low carrier-to-noise (C/N) characteristic of the received signal.
The effects of fading dips can be mitigated by having multiple receive antennas and employing some form of diversity combining, such as selective combining, equal gain combining, or maximal ratio combining, wherein signals from each receive antenna are combined to create a single received signal. Diversity techniques take advantage of the fact that the fading on the different antennas is not the same, so that when one antenna receives a fading dip, chances are the other antenna does not. Note Mobile Communications Design Fundamentals by William C. Y. Lee, Howard W. Sams & Co., Indiana, USA. In section 3.5.1 of this book, several examples are given describing how signals from two receiver amplifiers with separate antennas can be combined to counteract fading.
For longer path lengths, time dispersion occurs when the echoes are delayed with respect to the main ray. If an echo of sufficient magnitude arrives at the receiver delayed from the main ray by an amount of time on the order of the symbol period, time dispersion gives rise to intersymbol interference (ISI). Time dispersion may be advantageously corrected by using an equalizer. In the case of digital signal modulation, a maximum likelihood sequence estimation (MLSE) equalizer such as described in Digital Communications, 2.sup.nd Ed., by John G. Proakis, Mc-Graw Hill Book Company, New York, New York, USA, 1989 may be used. In section 6.7 of this book, various methods are described for detecting signals corrupted by time dispersion, or inter-symbol interference (ISI), using MLSE equalization.
There may also exist signal sources in the radio environment which are not orthogonal to the desired signal. Non-orthogonal signals, or interference, often come from radios operating on the same frequency (i.e., co-channel interference) or from radios operating on neighboring frequency bands (i.e., adjacent-channel interference). When the carrier-to-interference ratio (C/I) of a channel is too low, the quality of voice output at the mobile station is poor. Many techniques have been developed in order to minimize interference to tolerable levels including frequency re-use patterns and adaptive beamforming which can be used to steer a null in an antenna pattern in the direction of an interferer.
More recently, methods have been proposed that partially solve the problems of multipath fading and interference. In U.S. Pat. No. 5,191,598 to Backstrom, et al., for example, the problem of accurately detecting signals in the presence of fading and time dispersion is overcome by using a Viterbi-algorithm having a transmission function estimated for each antenna. By reference thereto, U.S. Pat. No. 5,191,598 is incorporated herein in its entirety. Another method of accurately detecting signals in the presence of fading and interference was presented in the IEEE Transactions on Vehicular Technology, Vol. 42, No. 4, Nov. 1993, J. H. Winters: "Signal Acquisition and Tracking with Adaptive Arrays in the Digital Mobile Radio System IS-54 with Flat Fading".
Although the above described conventional techniques can be used to improve signal quality, there remains room for improvement. Thus, in the parent application, interference rejection combining (IRC) techniques are described which combat interference, for example, using impairment correlations to improve the maximum likelihood sequence estimation.
However, the parent application describes techniques which can be used to improve the reception of signals. If used, for example, in a radio base station, these techniques will render the system unbalanced, i.e., the uplink will have superior quality to the downlink. If the system is unbalanced, then the system design will be predicated on the weakest link, i.e., the downlink, and cannot take full advantage of the increased quality provided by the IRC techniques used in the uplink. For example, if a system designer wanted to tradeoff improved quality for capacity by decreasing the frequency reuse, he or she would be hampered by the fact that the downlink quality was unimproved.